1. Field of the Invention
The present invention relates to a magnetoresistive element having a structure in which a current is supplied perpendicularly to the plane of the element, and a magnetic head, a magnetic recording/reproducing apparatus and a magnetic memory which use the magnetoresistive element.
2. Description of the Related Art
The performance of magnetic devices has drastically been improved by discovery of a giant magnetoresistive effect (GMR) in a stacked structure of magnetic films. In particular, a spin-valve film (SV film) has a structure easily applicable to a magnetic device and enables to effectively produce GMR effect. Consequently, the spin-valve film has brought about marked technical improvement to magnetic heads and magnetic devices such as MRAMs (magnetic random access memories).
The “spin-valve film” is a stacked film having a structure in which a nonmagnetic metal spacer layer is sandwiched between two ferromagnetic layers. In the spin-valve film, the magnetization of one ferromagnetic layer (referred to as a “pinned layer” or “magnetization pinned layer”) is pinned by an antiferromagnetic layer or the like, whereas the magnetization of the other ferromagnetic layer (referred to as a “free layer” or “magnetization free layer”) can be rotated according to an external field (for example, a media field). In the spin-valve film, a giant magnetoresistace change can be produced by varying a relative angle between the magnetization directions of the pinned layer and free layer.
Conventional spin-valve films are CIP (current-in-plane)-GMR elements in which a sense current is supplied parallel to the plane of the element. In recent years, much attention has been paid to CPP (current-perpendicular-to-plane)-GMR elements (referred to as “CPP elements” hereinafter) in which a sense current is supplied substantially perpendicular to the plane of the element.
When such a magnetoresistive element is applied to a magnetic head, a higher element resistance poses problems in regard to shot noise and high frequency response. In connection with these problems, the CPP element has an advantage that the element resistance depends on its area so that reduction in the size of the element increases the resistance change. The CPP element is thus advantageously applicable to magnetic devices on a trend of increasingly reducing the size thereof. Under the circumstances, the CPP element and the magnetic head using the same are expected to be promising candidates to achieve a recording density of 200 Gbpsi to 1 Tbpsi (terabits per square inch). However, CPP elements using a spacer layer made of a nonmagnetic metal exhibit only a very small resistance change. The CPP elements are thus hard to provide high output signals.
To partly solve this problem, an improved type of CPP (current-confined-path) element has been proposed which uses a spacer layer comprising an insulating layer in which fine current paths (current confined paths) formed of a nonmagnetic metal penetrating the insulating layer are formed. Such a CPP element (referred to as a CCP-CPP element hereinafter) exhibits a current confining effect and provides higher output signals than a simple CPP element using a nonmagnetic metal spacer layer. However, if the CCP-CPP element is applied to a magnetic head adapted for high density recording, the MR ratio thereof might still be insufficient.
Thus, a magnetoresistive element operating under a novel mechanism is greatly desired which realizes a very high MR ratio corresponding to a higher recording density.